1. Smart Pointers

2. Dumb Pointers Pointers Necessary – Dynamic memory – Sharing memory

3. Dumb Pointers Pointers Necessary – Dynamic memory – Sharing memory But they are tricky – Memory leaks – Bad pointers

4. References References : safer pointers – C++ : Non-nullable

5. References Java : – All objects are on heap – Stack has Numeric Variables References – References are nullable – Cannot delete memory

6. References Python Everything is a reference… even numbers x = 1 x 1

7. References Python Everything is a reference… even numbers x = 1 x = 2 x 1 2

8. References Python Everything is a reference… even numbers x = 1 x = 2 y = x x 1 2 y

9. References Python Everything is a reference… even numbers x = 1 x = 2 y = x x = 4 x 1 2 y 4

10. References Python Everything is a reference… even numbers x = 1 x = 2 y = x x = 4 x = x + 1 x 1 2 y 4 5

11. Garbage Collection Garbage collection: Automatically reclaim unused memory from heap Various strategies Reference counting Mark and Sweep

12. Heap of Fish http://web.informatik.uni- bonn.de/IV/martini/Lehre/Veranstaltungen/SS00/InformatikII /JavaSimulation/HeapOfFish.html http://web.informatik.uni- bonn.de/IV/martini/Lehre/Veranstaltungen/SS00/InformatikII /JavaSimulation/HeapOfFish.html

13. Compaction Yellow fish are garbage:

14. Still Problem Still can't make a new Red Fish

15. Much Better Compacting "live" objects reduces fragmentation:

16. Smart Pointers C++11 brought smart pointers – Pointers do reference counting – Memory released when last reference is released

17. Shared Ptr shared_ptr : counts number sharing the object – When shared_ptr leaves scope, count-- – Memory deleted when count == 0

18. Weak Ptr weak_ptr : keeps track of object without "counting" – Can not be used directly – Use to ask for a real shared ptr when needed – Used to avoid cycles in shared_ptr

19. Unique Ptr unique_ptr : unshared pointer – Can't copy can only move – Deletes memory when pointer leaves scope

20. Courses With Strong/Weak Course Manager responsible for loading from DB

21. Courses With Strong/Weak Student asks for pointers to courses Course manager creates objects, stores weak_ptr

22. Courses With Strong/Weak Student asks for pointers to courses Student given shared_ptrs

23. Courses With Strong/Weak Student asks for pointers to courses Student given shared_ptrs

24. Courses With Strong/Weak Student asks for pointers to courses New student asks for a new class

25. Courses With Strong/Weak Student asks for pointers to courses New student asks for an existing class CS162 now has refcount of 2

26. Courses With Strong/Weak Student asks for pointers to courses First student destroyed… shared_ptrs with refcount == 1 destroyed

27. Objective C Can pick: – Manual release – Garbage collection (on some platforms) – ARC : Automatic Reference Counting Like shared_ptrs and weak_ptrs